Insulated dc/dc converter and primary side controller thereof, control method, and power adaptor and electronic device using the same

ABSTRACT

A primary side controller for controlling a switching transistor on a primary side of an insulated DC/DC converter, includes: a low voltage state detecting circuit configured to detect a low voltage state in which an output voltage of the DC/DC converter is lower than a predetermined value; and a pulse width modulator configured to generate a pulse signal whose ON time is adjusted depending on a feedback signal from a secondary side, wherein a period of the pulse signal in the low voltage state is longer than a period of the pulse signal in a non-low voltage state in which the output voltage is higher than the predetermined value.

CROSS-REFERENCE TO RELATED APPLICATION

The present invention claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2016-135696, filed on Jul. 8, 2016, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an insulated DC/DC converter.

BACKGROUND

An insulated DC/DC converter, specifically, a flyback type or forward type DC/DC converter, is used in various power source circuits including an AC/DC converter. FIG. 1 is a circuit diagram of an AC/DC converter 100 r including a synchronous rectification type flyback converter 200 r.

The AC/DC converter 100 r mainly includes a fuse 102, an input capacitor Ci, a filter 104, a diode rectifier circuit 106, a smoothing capacitor Cs, and the flyback converter 200 r.

A commercial AC voltage V_(AC) is input to the filter 104 via the fuse 102 and the input capacitor Ci. The filter 104 removes the noise of the commercial AC voltage V_(AC). The diode rectifier circuit 106 is a diode bridge circuit that full-wave rectifies the commercial AC voltage V_(AC). An output voltage from the diode rectifier circuit 106 is smoothed by the smoothing capacitor Cs and converted into a DC voltage V_(IN).

The insulated flyback converter 200 r receives the DC voltage V_(IN) by an input terminal P1, steps down the same, and supplies an output voltage V_(OUT) stabilized to a target value to a load (not shown) connected to an output terminal P2.

A switching transistor M1 is connected to a primary winding W1 of a transformer T1. A rectifier diode D1 is connected to a secondary winding W2 of the transformer T1, and an output capacitor Co1 is connected to the output terminal P2. A feedback circuit 206 drives a light emitting element of a photocoupler 204 by a current corresponding to a difference between the output voltage V_(OUT) and its target voltage V_(OUT(REF)). A feedback current I_(FB) corresponding to the difference flows through a light receiving element of the photocoupler 204.

A rectifier diode D2 and a smoothing capacitor Co2 form a power source circuit 208 together with an auxiliary winding W3 of the transformer T1. A source voltage V_(CC) generated by the power source circuit 208 is supplied to a power (VCC) terminal of a primary side controller 300 r.

The primary side controller 300 r is a pulse width modulation (PWM) controller. A feedback signal V_(FB)corresponding to the feedback current I_(FB) is generated at a feedback (FB) terminal of the primary side controller 300 r. Further, a current detection signal V_(CS) proportional to a primary current I_(P) flowing through the switching transistor M1 is feedback to a current detection (CS) terminal of the primary side controller 300 r. The current detection signal V_(CS) is a voltage drop of a sense resistor R_(CS) installed in series with the switching transistor M1.

For example, the primary side controller 300 r includes a pulse width modulator of a peak current mode, and generates a pulse signal S_(OUT) having a constant frequency (period) and having a duty ratio corresponding to the feedback signal V_(FB)and the current detection signal V_(CS) to drive the switching transistor M1 connected to an output (OUT) terminal.

The present inventor has reviewed an operation of the DC/DC converter 200 r of FIG. 1 in a state where the output voltage V_(OUT) is low, and recognized the following issues. The output voltage V_(OUT) is low, for example, immediately after the startup or when the output of the DC/DC converter 200 r is grounded.

FIG. 2 is a waveform diagram after the start of the DC/DC converter 200 r of FIG. 1. The switching transistor M1 is turned on in synchronization with an oscillator clock (a set signal S_(SET)) of a constant period T_(OSC). When the switching transistor M1 is turned on, the primary current I_(P) flows through the primary winding W1. The primary current I_(P) increases with a slope V_(IN)/L_(P) proportional to an input voltage V_(IN). L_(P) is the inductance of the primary winding W1.

When the primary current I_(P) reaches a peak level corresponding to the feedback signal V_(FB), the switching transistor M1 is turned off. During an OFF period of the switching transistor M1, a secondary current I_(S) flows through the secondary winding W2. The secondary current I_(S) decreases with a slope (V_(OUT)+V_(F))/L_(S). L_(S) denotes the inductance of the secondary winding W2 and V_(F) denotes a forward voltage of the rectifier diode D1.

Thereafter, when the set signal S_(SET) is asserted, the switching transistor M1 is turned on again. The primary current I_(P) starts to rise with a current amount of the secondary current I_(S) immediately before the switching transistor M1 is turned on, as a starting point. The DC/DC converter 200 r repeats this operation.

A decrement ΔI_(S) of the secondary current I_(S) in an OFF time T_(OFF) is ΔI_(S)=(V_(OUT)+V_(F))/L_(S)×T_(OFF). Since the output voltage V_(OUT) is low immediately after the startup, the slope of the secondary current I_(S) in the OFF time T_(OFF) is small, and therefore, the decrement ΔI_(S) is small. Thus, the secondary current I_(S) does not drop to zero during the OFF time T_(OFF).

When this operation repeats, the DC/DC converter 200 r starts in a continuous mode and large currents I_(P) and I_(S) flow through the primary winding W1 and the secondary winding W2. When the DC/DC converter 200 r starts in the continuous mode, a very high surge voltage exceeding 100V is generated across the rectifier diode D1 of the secondary side. In addition, a drain voltage V_(DS1) of the switching transistor M1 increases. Further, the same phenomenon occurs not only at the startup but also when the output is grounded.

Therefore, in order to cope with the operation in the current continuation mode in a low output state, conventionally, it was necessary to devise the transformer T1 or to design a withstand voltage and the like of the switching transistor M1 or the rectifier diode D1 to be high, leading to an increase in cost.

SUMMARY

Some embodiments of the present disclosure provide a DC/DC converter in which a current in a low voltage state is suppressed.

According to one embodiment of the present disclosure, there is provided a primary side controller for controlling a switching transistor on a primary side of an insulated DC/DC converter. The primary side controller includes a low voltage state detecting circuit configured to detect a low voltage state in which an output voltage of the DC/DC converter is lower than a predetermined value, and a pulse width modulator configured to generate a pulse signal having an ON time which is adjusted depending on a feedback signal from a secondary side of insulated DC/DC converter. A period of the pulse signal in the low voltage state is longer than a period of the pulse signal in a non-low voltage state in which the output voltage is higher than the predetermined value.

By lengthening the period of the pulse signal in the low voltage state, an OFF time of the switching transistor is lengthened. Thus, it is possible to increase a decrement of a current in the secondary winding, and to reduce a current in the low voltage state.

Further, according to another embodiment of the present disclosure, a primary side controller is provided. The primary side controller is for controlling a switching transistor on a primary side of an insulated DC/DC converter, and it includes a low voltage state detecting circuit configured to detect a low voltage state in which an output voltage of the DC/DC converter is lower than a predetermined value, and a pulse width modulator configured to generate a pulse signal having an ON time which is adjusted depending on a feedback signal from a secondary side of the insulated DC/DC converter. An OFF time of the pulse signal in the low voltage state is longer than an OFF time of the pulse signal in a non-low voltage state in which the output voltage is higher than the predetermined value

By lengthening the OFF time of the switching transistor in the low voltage state, it is possible to increase the decrement of the current in the secondary winding, and to reduce the current flowing through the transformer in the low voltage state.

Further, according to another embodiment of the present disclosure, a primary side controller is provided. The primary side controller includes a low voltage state detecting circuit configured to detect a low voltage state in which an output voltage of the DC/DC converter is lower than a predetermined value, and a pulse width modulator configured to generate a pulse signal having an ON time which is adjusted depending on a feedback signal from a secondary side of the insulated DC/DC converter. A timing, at which the pulse signal transitions to an ON level in the low voltage state, is delayed, as compared with a timing of a non-low voltage state in which the output voltage is higher than the predetermined value

Thus, it is possible to lengthen the OFF time of the switching transistor in the low voltage state, to increase the decrement of the current in the secondary winding becomes larger, and to the current flowing through the transformer in the low voltage state.

The pulse width modulator may include a reset signal generating circuit configured to generate a reset signal asserted depending on the feedback signal, a set signal generating circuit configured to generate a periodic set signal, and a logic circuit configured to generate the pulse signal which transitions to an ON level in response to the set signal and to an OFF level in response to the reset signal. Further, a period of the set signal in the low voltage state may be longer than a period of the set signal in the non-low voltage state.

The set signal generating circuit may include an oscillator configured to oscillate at a predetermined frequency, and a counter configured to set an M period (where M is a natural number) of an oscillation signal from the oscillator in the non-low voltage state as the period of the set signal, and to set an N period (where N>M) of the oscillation signal in the low voltage state as the period of the set signal.

The set signal generating circuit may include a variable frequency oscillator, and the oscillation frequency of the oscillator may be dependent on a detection signal corresponding to the output voltage in the low voltage state.

The DC/DC converter may include a transformer including a primary winding, a secondary winding, and an auxiliary winding. The low voltage state detecting circuit may be configured to monitor the output voltage based on a voltage across the auxiliary winding at an OFF time of the switching transistor.

During the OFF time of the switching transistor, a voltage substantially proportional to the output voltage is generated in the auxiliary winding. Thus, it is possible to detect the low voltage state by monitoring a voltage of the auxiliary winding.

The switching transistor may be housed in the same package as that of the primary side controller. Reducing the current in the low voltage state reduces a voltage between a drain and a source of the switching transistor. Therefore, it is possible to ease the conditions such as a withstand voltage or the like required for the switching transistor, and it is easy to house it in the same package as that of the primary side controller.

The reset signal generating circuit may include a reset comparator configured to compare a current detection signal indicating a current flowing through the switching transistor with the feedback signal. The reset signal may be dependent on an output signal of the reset comparator.

Further, according to another embodiment of the present disclosure, a primary side controller is provided. The primary side controller includes a low voltage state detecting circuit configured to detect a low voltage state in which an output voltage of the DC/DC converter is lower than a predetermined value, and a pulse width modulator configured to generate a narrow pulse signal having an ON time which is independent of the feedback signal in the low voltage state, and to generate a pulse signal having an ON time which is adjusted depending on a feedback signal from a secondary side of the insulated DC/DC converter in a non-low voltage state in which the output voltage is higher than the predetermined value.

By narrowing the pulse width of the pulse signal in the low voltage state, namely the ON time, it is possible to reduce an increment of the current of the primary winding during the ON time. Thus, it is possible to reduce the current in the low voltage state.

According to another embodiment of the present disclosure, there is provided a DC/DC converter. The DC/DC converter is an isolated DC/DC converter, which includes a transformer including a primary winding, a secondary winding, and an auxiliary winding, a switching transistor connected to the primary winding, a secondary side rectifier circuit configured to rectify a current flowing through the secondary winding to generate an output voltage, a photocoupler including a light emitting element and a light receiving element, a feedback circuit configured to drive the light emitting element of the photocoupler with a current corresponding to a difference between the output voltage and a target value, and a primary side controller configured to drive the switching transistor based on a feedback signal corresponding to a current flowing through the light receiving element of the photocoupler. The primary controller is configured to drive (i) the switching transistor depending on a first pulse signal which has a first period and an ON time of the first pulse signal is adjusted depending on the feedback signal when a voltage across the auxiliary winding is greater than a predetermined value, and drive (ii) the switching transistor depending on a second pulse signal which has a second period longer than the first period and an ON time of the second pulse signal is adjusted depending on the feedback signal when the voltage across the auxiliary winding is smaller than the predetermined value.

According to another embodiment of the present disclosure, an electronic device is provided. The electronic device may include a load, a diode rectifier circuit configured to full-wave rectify a commercial AC voltage, a smoothing capacitor configured to smooth an output voltage of the diode rectifier circuit to generate a DC input voltage, and any one of the aforementioned DC/DC converters configured to step down the DC input voltage and supplies the same to the load.

According to another embodiment of the present disclosure, a power adaptor is provided. The power adaptor may include a diode rectifier circuit configured to full-wave rectify a commercial AC voltage, a smoothing capacitor configured to smooth an output voltage of the diode rectifier circuit to generate a DC input voltage, and any one of the aforementioned DC/DC converters configured to step down the DC input voltage and supplies the same to a load.

According to another embodiment of the present disclosure, there is provided a method of controlling a switching transistor on a primary side of an isolated DC/DC converter. The method of controlling includes comparing an output voltage of the DC/DC converter with a predetermined value, generating a first pulse signal having ON time which is adjusted depending on a feedback signal from a secondary side of the isolated DC/DC converter, the first pulse signal having a first period when the output voltage is higher than the predetermined value, generating a second pulse signal having an ON time which is adjusted depending on the feedback signal from the secondary side, the second pulse signal having a second period longer than the first period when the output voltage is lower than the predetermined value, and driving the switching transistor depending on the first or second pulse signal.

According to another embodiment of the present disclosure, a method of controlling is provided. The method of controlling includes comparing an output voltage of the DC/DC converter with a predetermined value, generating a first pulse signal having an ON time which is adjusted depending on a feedback signal from a secondary side of the isolated DC/DC converter, the first pulse signal having a first period when the output voltage is higher than the predetermined value, generating a second pulse signal having an ON time which is adjusted depending on the feedback signal from the secondary side, the second pulse signal having an OFF time which is longer than the first period when the output voltage is lower than the predetermined value; and driving the switching transistor depending on the first or second pulse signal.

In addition, any combination of the foregoing components or any replacement of the components or representations of the present disclosure between methods, devices, systems, and the like may also be effective as embodiments of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an AC/DC converter including a synchronous rectification type flyback converter.

FIG. 2 is a waveform diagram when the DC/DC converter of FIG. 1 starts.

FIG. 3 is a block diagram of a DC/DC converter according to an embodiment of the present disclosure.

FIG. 4A is an operational waveform diagram of a DC/DC converter in a non-low voltage state, and FIG. 4B is an operational waveform diagram of a DC/DC converter in a low voltage state.

FIG. 5 is a circuit diagram illustrating a configuration example of a primary side controller.

FIG. 6 is a diagram illustrating a configuration example of a set signal generating circuit.

FIG. 7A is a block diagram of a low voltage state detecting circuit and a set signal generating circuit according to a first modification, and FIG. 7B is a diagram illustrating a relationship between an output voltage V_(OUT) and a frequency f_(OSC) of a set signal S_(SET).

FIG. 8 is a circuit diagram of a primary side controller according to a second modification.

FIG. 9 is a diagram illustrating an AC adapter including an AC/DC converter.

FIGS. 10A and 10B are diagrams illustrating an electronic device including an AC/DC converter.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be now described in detail with reference to the drawings. Like or equivalent components, members, and processes illustrated in each drawing are given like reference numerals and a repeated description thereof will be properly omitted. Further, the embodiments are presented by way of example only, and are not intended to limit the present disclosure, and any feature or combination thereof described in the embodiments may not necessarily be essential to the present disclosure.

In the present disclosure, “a state where a member A is connected to a member B” includes a case where the member A and the member B are physically directly connected or even a case where the member A and the member B are indirectly connected through any other member that does not affect an electrical connection state between the members A and B or does not impair functions and effects achieved by combinations of the members A and B.

Similarly, “a state where a member C is installed between a member A and a member B” includes a case where the member A and the member C or the member B and the member C are indirectly connected through any other member that does not affect an electrical connection state between the members A and C or the members B and C or does not impair function and effects achieved by combinations of the members A and C or the members B and C, in addition to a case where the member A and the member C or the member B and the member C are directly connected.

FIG. 3 is a block diagram of a DC/DC converter 200 according to an embodiment of the present disclosure. The DC/DC converter 200 includes a transformer T1, a primary side controller 300, a secondary side rectifier circuit 202, a photocoupler 204, a feedback circuit 206, and a power source circuit 208.

The transformer T1 includes a primary winding W1, a secondary winding W2, and an auxiliary winding W3. The switching transistor M1 is connected to the primary winding W1. In this embodiment, the switching transistor M1 is housed in a package of the primary side controller 300.

The secondary side rectifier circuit 202 includes a rectifier diode D1 and an output capacitor Co1, and rectifies a current I_(S) flowing through the secondary winding W2 to generate an output voltage V_(OUT). The photocoupler 204 includes a light emitting element and a light receiving element. The feedback circuit 206 is, for example, a shunt regulator, and drives the light emitting element of the photocoupler 204 by a current corresponding to a difference between the output voltage V_(OUT) and its target value V_(OUT(REF)). A feedback current I_(FB) corresponding to the difference between the output voltage V_(OUT) and the target value V_(OUT(REF)) flows through the light receiving element of the photocoupler 204.

The primary side controller 300 includes a source (SOURCE) terminal, a drain (DRAIN) terminal, a feedback (FB) terminal, a power (VCC) terminal, and a voltage detection (ZT) terminal, and is housed in a single package.

The FB terminal is connected to the light receiving element of the photocoupler 204, and a feedback signal V_(FB) corresponding to the feedback current I_(FB) is generated at the FB terminal. A drain of the switching transistor M1 is connected to the DRAIN terminal, and a source terminal of the switching transistor M1 is connected to the SOURCE terminal. A current sensor resistor R_(CS) is externally attached between the SOURCE terminal and a ground. A voltage drop (current detection signal) V_(CS) proportional to the current I_(S) flowing through the primary winding W1 is generated in the current sense resistor R_(CS) during an ON time of the switching transistor M1. The current detection signal V_(CS) is used for overcurrent protection and also used to adjust the ON time. Further, the current sense resistor R_(CS) may be embedded in the primary side controller 300.

The primary side controller 300 switches the switching transistor M1 depending on the feedback signal V_(FB)such that the difference between the output voltage V_(OUT) and the target value V_(OUT(REF)) approaches zero. The primary side controller 300 includes a pulse width modulator 302, a driver 304, and a low voltage state detecting circuit 310, in addition to the switching transistor M1.

The low voltage state detecting circuit 310 detects a low voltage state in which the output voltage V_(OUT) of the DC/DC converter 200 is lower than a predetermined value V_(L). Further, a state in which the output voltage V_(OUT) is higher than the predetermined value V_(L) will be referred to as a non-low voltage state. Although not limited thereto, for example, a voltage V_(W3) across the auxiliary winding W3 may be used to detect the low voltage state. During an OFF time of the switching transistor M1, a voltage (V_(OUT)+V_(F))×n_(A)/n_(S) substantially proportional to the output voltage V_(OUT) appears in the auxiliary winding W3. V_(F) is a forward voltage of the rectifier diode D1, and n_(A) is the number of turns of the auxiliary winding W3 and n_(S) is the number of turns of the secondary winding W2.

The voltage V_(W3) of the auxiliary winding W3 is divided by resistors R₂₁ and R₂₂ and input to the ZT terminal. The low voltage stage detecting circuit 310 detects a low voltage state by comparing a voltage V_(ZT) of the ZT terminal with a threshold voltage V_(TH) corresponding to the predetermined value V_(L). A determination signal S1 indicating whether it is in a low voltage state or a non-low voltage state is input to the pulse width modulator 302 from the low voltage state detecting circuit 310. The determination signal S1 may be a signal indicating a degree of a low voltage. Further, the ZT terminal should not be confused with a terminal for detecting a zero current installed in a quasi-resonance controller.

The pulse width modulator 302 generates a pulse signal S_(PWM) at least depending on the feedback signal V_(FB). The driver 304 drives the switching transistor M1 depending on the pulse signal S_(PWM).

The pulse width modulator 302 adjusts an ON time of the pulse signal S_(PWM) depending on the feedback signal V_(FB) from the secondary side. Although not limited thereto, the pulse width modulator 302 is configured to shift the pulse single S_(PWM) to an OFF level when the current detection signal V_(CS) reaches the feedback signal V_(FB). Further, the pulse width modulator 302 is configured such that a period T_(P2) of the pulse signal S_(PWM) in a low voltage state is longer than a period T_(P1) of the pulse signal S_(PWM) in a non-low voltage state. The period T_(P1) may be determined such that the current flowing through the transformer T1 becomes a desired amount in the non-low voltage state, and is defined, for example, such that the DC/DC converter 200 is in a discontinuous current mode (or a critical mode).

The period T_(P2) may be determined such that the current flowing through the transformer T1 becomes a desired amount in the low voltage state, and similar to the non-low voltage state, it is defined such that the DC/DC converter 200 is in a discontinuous current mode (or a critical mode).

That is, the primary side controller 300 drives (i) the switching transistor M1 depending on the pulse signal S_(PWM) which has the first period T_(P1) and whose ON time is adjusted depending on the feedback signal V_(FB) when the voltage V_(W3) across the auxiliary winding W3 is greater than the predetermined value V_(L). Further, the primary side controller 300 drives (ii) the switching transistor M1 depending on the pulse signal S_(PWM) which has the second period T_(P2) longer than the first period T_(P1) and whose ON time is adjusted depending on the feedback signal V_(FB) when the voltage V_(W3) across the auxiliary winding W3 is smaller than the predetermined value V_(L).

The above is the configuration of the DC/DC converter 200. Next, an operation thereof will be described.

FIG. 4A is an operational waveform diagram of the DC/DC converter 200 in a non-low voltage state. When the output voltage V_(OUT) is higher than the predetermined value V_(L), the period of the pulse signal S_(PWM) becomes a first value (first period) T_(P1). Since the output voltage V_(OUT) is sufficiently high, the slope of the current I_(S) of the secondary winding W2 at the OFF time T_(OFF) is sharp, and thus, the DC/DC converter 200 operates in the discontinuous current mode.

FIG. 4B is an operational waveform diagram of the DC/DC converter 200 in a low-voltage state. When the output voltage V_(OUT) is lower than the predetermined voltage V_(L), the period of the pulse signal S_(PWM) becomes a second value (second period) T_(P2). Since the output voltage V_(OUT) is low, the slope of the current I_(S) of the secondary winding W2 at the OFF time T_(OFF) is gentle. Conventionally, since the OFF time is short, the DC/DC converter 200 r operated in the continuous current mode as illustrated in FIG. 2. In FIG. 4B, by setting the period T_(P2) longer than the period T_(P1) of the non-low voltage state, the OFF time T_(OFF2) in the low voltage state becomes sufficiently longer than the OFF time of the non-low voltage state. Therefore, during the OFF time T_(OFF), the current I_(S) of the secondary winding W2 can be lowered to a sufficiently small amount (zero in FIG. 4B).

In this manner, in the DC/DC converter 200 according to the embodiment, it is possible to reduce the current flowing through the transformer in the low voltage state. Accordingly, since the transformer T1, the switching transistor M1, or the rectifier diode D1 having a low withstand voltage can be employed, the cost can be lowered. In particular, since the switching transistor M1 has a low withstand voltage, the switching transistor M1 can be housed in the same package as that of the primary side controller 300.

The present disclosure is recognized by the block diagram or the circuit diagram of FIG. 3 or intended to cover various devices and circuits derived from the above description, but is not limited to the specific configuration. Hereinafter, a more specific configuration example or an example will be described in order to help understand and clarify the essence of the present disclosure and a circuit operation thereof, rather than to narrow the scope of the present disclosure.

FIG. 5 is a circuit diagram illustrating a configuration example of the primary side controller 300. The low voltage state detecting circuit 310 includes a comparator 312. The comparator 312 compares a voltage V_(ZT) of the ZT terminal with a threshold voltage V_(TH) corresponding to a predetermined value V_(L), and generates a comparison signal S2 which has a high level when V_(ZT) is greater than V_(TH) (V_(ZT)>V_(TH)), i.e., in the non-low voltage state (V_(OUT)>V_(L)), and which has a low level when V_(ZT) is smaller than V_(TH) (V_(ZT)<V_(TH)), i.e., in the low voltage state (V_(OUT)<V_(L)). The low voltage state detecting circuit 310 outputs a determination signal S1 corresponding to the comparison signal S2. Since V_(ZT) is proportional to V_(OUT) during an OFF time of the switching transistor M1, the logic circuit 314 may latch the comparison signal S2 during the OFF time at every cycle as the determination signal S2.

The pulse width modulator 302 includes a set signal generating circuit 320, a reset signal generating circuit 322, and a logic circuit 324. The set signal generating circuit 320 generates a set signal S_(SET) asserted (e.g., having a high level) at every period of the pulse signal S_(PWM). The period of the set signal S_(SET) may be changed depending on the determination signal S1 and is T_(P1) in the non-low voltage state and T_(P2) in the low voltage state. The set signal S_(SET) is a signal defining a timing at which the pulse signal S_(PWM) transitions to an ON level, i.e., at a timing at which the switching transistor M1 is turned on. Therefore, increasing the period of the set signal S_(SET) is equivalent to delaying a timing at which the switching transistor M1 is turned on, i.e., lengthening the OFF time T_(OFF).

The reset signal generating circuit 322 generates a reset signal S_(RESET) asserted (having a high level) depending on the feedback signal V_(FB). The reset signal generating circuit 322 includes a PWM comparator 326. The PWM comparator 326 compares the current detection signal V_(CS) with the feedback signal V_(FB), and when V_(CS) is greater than V_(FB) (V_(CS)>V_(FB)), the PWM comparator 326 asserts a comparison signal S3 (e.g., a high level). A current limit comparator 328 is installed for pulse-by-pulse overcurrent protection. The current limit comparator 328 compares the current detection signal V_(CS) with a threshold V_(CL) defining an allowable peak value of the current I_(S), and when V_(CS) is greater than V_(CL) (V_(CS)>V_(CL)), the current limit comparator 328 asserts a comparison signal S4 (e.g., a high level). When at least one of the comparison signals S3 and S4 is asserted, an OR gate 330 asserts the reset signal S_(RESET).

The logic circuit 324 generates the pulse signal S_(PWM) which shifts to an ON level (high level) in response to the set signal S_(SET) and shifts to an OFF level (low level) in response to the reset signal S_(RESET). The logic circuit 324 may be configured as an SR flipflop or a D flipflop.

FIG. 6 is a diagram illustrating a configuration example of a set signal generating circuit 320 a. The set signal generating circuit 320 a of FIG. 6 includes an oscillator 332 and a counter 334. The oscillator 332 oscillates at a predetermined frequency. In the non-low voltage state, the counter 334 sets an M period (where M is a natural number) of an oscillation signal from the oscillator 332 as the period T_(P1) of the set signal S_(SET), wherein M may be 1.

T _(P1) =T _(OSC) ×M

In the low voltage state, the counter 334 sets an N period (where N>M) of an oscillation signal S5 as the period T_(P2) of the set signal S_(SET).

T _(P2) =T _(OSC) ×N

In modifications, the set signal generating circuit may include an oscillator whose oscillation frequency (period) may be a variable frequency switchable depending on the determination signal S1. The oscillator is a voltage controlled oscillator (VCO), and a control voltage may be switched depending on the determination signal S1. Alternatively, the oscillator 332 may include a PLL circuit and a frequency division ratio may be switched depending on the determination signal S1.

The present disclosure has been described above based on the embodiment. It is to be understood by those skilled in the art that the embodiment is merely illustrative and may be differently modified by any combination of the components or processes, and the modifications are also within the scope of the present disclosure. Hereinafter, these modifications will be described.

First Modification

In the embodiment, the period T_(P2) of the pulse signal S_(PWM) in the low voltage state is constant, but it may be changed depending on the output voltage V_(OUT). FIG. 7A is a block diagram of a low voltage state detecting circuit 310 and a set signal generating circuit 320 according to a first modification. The low voltage state detecting circuit 310 includes a sample hold circuit 316 and an amplifier 318. The sample hold circuit 316 samples and holds a voltage V_(ZT) of the ZT terminal at an OFF time of the switching transistor M1. The amplifier 318 amplifies an output voltage V1 from the sample hold circuit 316 as necessary. The set signal generating circuit 320 oscillates at a frequency f_(OSC) corresponding to an output voltage V2 from the amplifier 318. FIG. 7B is a diagram illustrating a relationship between the output voltage V_(OUT) and a frequency f_(OSC) of the set signal S_(SET).

Second Modification

In the embodiment, the discontinuous current mode in the low voltage state is realized by lengthening an OFF time by changing the period of the pulse signal S_(PWM). On the other hand, in a second modification, the discontinuous current mode is realized by shortening an ON time in the low voltage state, while keeping the period of the pulse signal S_(PWM) constant. FIG. 8 is a circuit diagram of a primary side controller 300 b according to the second modification. A pulse width modulator 302 b generates a pulse signal S_(PWM) whose ON time T_(ON) is adjusted depending on a feedback signal from the secondary side in the non-low voltage state. Further, the pulse width modulator 302 b generates a narrow pulse signal whose ON time is independent of the feedback signal V_(FB), in the low voltage state. A forcible reset circuit 340 asserts a forcible reset signal S6 after the lapse of a predetermined period of time from assertion of the set signal S_(SET) in the low voltage state. The forcible reset signal S6 is input to a logic circuit 324 through an OR gate 342.

According to this modification, by setting a timing at which the switching transistor M1 is turned off earlier than the reset signal generated by the PWM comparator 326, it is possible to generate a pulse signal whose ON time is short and narrow. By switching the switching transistor M1 with the narrow pulse signal, since the increment of the current I_(S) of the primary winding W1 at the ON time of the switching transistor M1 can be reduced, it is possible to reduce a current flowing through the transformer in the low voltage state.

Third Modification

In the embodiment, the low voltage state detecting circuit 310 monitors a voltage of the auxiliary winding W3, but the present disclosure is not limited thereto. Since the FB signal indicates a difference between the output voltage V_(OUT) and the target voltage, it may be said to indirectly indicate the output voltage V_(OUT). Therefore, the low voltage state detecting circuit 310 may monitor a low voltage state using the FB signal. Alternatively, a comparator for comparing the output voltage V_(OUT) with the threshold V_(L) may be disposed on the secondary side of the transformer T1 and an output of the comparator may be transmitted from a photocoupler different from the photocoupler 204 to the primary side controller 300.

Fourth Modification

In the embodiment, the diode rectifier type DC/DC converter has been described, but the present disclosure is also applicable to a synchronous rectification type DC/DC converter. Further, the switching transistor M1 may be installed outside the primary side controller 300.

Applications

Finally, applications of the DC/DC converter 200 will be described. The DC/DC converter 200 can be used for the AC/DC converter 100 suitably used for a power source block of an AC adapter or an electronic device.

FIG. 9 is a diagram illustrating an AC adapter 800 including the AC/DC converter 100. The AC adapter 800 includes a plug 802, a housing 804, and a connector 806. The plug 802 receives a commercial AC voltage V_(AC) from an outlet (not shown). The AC/DC converter 100 is mounted in the housing 804. A DC output voltage V_(OUT) generated by the AC/DC converter 100 is supplied from the connector 806 to the electronic device 810. Examples of the electronic device 810 include a notebook PC, a digital camera, a digital video camera, a mobile phone, a portable audio player, and the like.

FIGS. 10A and 10B are diagrams illustrating an electronic device 900 including the AC/DC converter 100. The electronic device 900 of FIGS. 10A and 10B is a display device, but the type of the electronic device 900 is not particularly limited thereto, and it may be a device, which incorporates a power source device, such as an audio device, a refrigerator, a washing machine, a vacuum cleaner, or the like.

The plug 902 receives a commercial AC voltage V_(AC) from an outlet (not shown). The AC/DC converter 100 is mounted in a housing 904. A DC output voltage V_(OUT) generated by the AC/DC converter 100 is supplied to a load, which is mounted in the same housing 904, such as a microcomputer, a digital signal processor (DSP), a power source circuit, a lighting device, an analog circuit, a digital circuit, or the like.

According to the present disclosure in some embodiments, it is possible to reduce a current flowing through a transformer in a low voltage state.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosures. Indeed, the embodiments described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the disclosures. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosures. 

What is claimed is:
 1. A primary side controller for controlling a switching transistor on a primary side of an insulated DC/DC converter, comprising: a low voltage state detecting circuit configured to detect a low voltage state in which an output voltage of the DC/DC converter is lower than a predetermined value; and a pulse width modulator configured to generate a pulse signal having an ON time which is adjusted depending on a feedback signal from a secondary side of the insulated DC/DC converter, wherein a period of the pulse signal in the low voltage state is longer than a period of the pulse signal in a non-low voltage state in which the output voltage is higher than the predetermined value.
 2. A primary side controller for controlling a switching transistor on a primary side of an insulated DC/DC converter, comprising: a low voltage state detecting circuit configured to detect a low voltage state in which an output voltage of the DC/DC converter is lower than a predetermined value; and a pulse width modulator configured to generate a pulse signal having an ON time which is adjusted depending on a feedback signal from a secondary side of the insulated DC/DC converter, wherein an OFF time of the pulse signal in the low voltage state is longer than an OFF time of the pulse signal in a non-low voltage state in which the output voltage is higher than the predetermined value.
 3. A primary side controller for controlling a switching transistor on a primary side of an insulated DC/DC converter, comprising: a low voltage state detecting circuit configured to detect a low voltage state in which an output voltage of the DC/DC converter is lower than a predetermined value; and a pulse width modulator configured to generate a pulse signal having an ON time which is adjusted depending on a feedback signal from a secondary side of the insulated DC/DC converter, wherein a timing, at which the pulse signal transitions to an ON level in the low voltage state, is delayed, as compared with a timing of a non-low voltage state in which the output voltage is higher than the predetermined value.
 4. The controller of claim 1, wherein the pulse width modulator comprises: a reset signal generating circuit configured to generate a reset signal asserted depending on the feedback signal; a set signal generating circuit configured to generate a periodic set signal; and a logic circuit configured to generate the pulse signal which transitions to an ON level in response to the set signal and to an OFF level in response to the reset signal, wherein a period of the set signal in the low voltage state is longer than a period of the set signal in the non-low voltage state.
 5. The controller of claim 4, wherein the set signal generating circuit comprises: an oscillator configured to oscillate at a predetermined frequency; and a counter configured to set an M period (where M is a natural number) of an oscillation signal from the oscillator in the non-low voltage state as the period of the set signal, and to set an N period (where N>M) of the oscillation signal in the low voltage state as the period of the set signal.
 6. The controller of claim 4, wherein the set signal generating circuit includes a variable frequency oscillator, and wherein the oscillation frequency of the oscillator is dependent on a detection signal corresponding to the output voltage in the low voltage state.
 7. The controller of claim 1, wherein the DC/DC converter comprises a transformer including a primary winding, a secondary winding, and an auxiliary winding, and wherein the low voltage state detecting circuit is configured to monitor the output voltage based on a voltage across the auxiliary winding at an OFF time of the switching transistor.
 8. The controller of claim 1, wherein the switching transistor is housed in the same package as that of the primary side controller.
 9. The controller of claim 4, wherein the reset signal generating circuit includes a reset comparator configured to compare a current detection signal indicating a current flowing through the switching transistor with the feedback signal, and wherein the reset signal is dependent on an output signal of the reset comparator.
 10. A primary side controller for controlling a switching transistor on a primary side of an insulated DC/DC converter, comprising: a low voltage state detecting circuit configured to detect a low voltage state in which an output voltage of the DC/DC converter is lower than a predetermined value; and a pulse width modulator configured to generate a pulse signal having an ON time which is adjusted depending on a feedback signal from a secondary side of the insulated DC/DC converter in a non-low voltage state in which the output voltage is higher than the predetermined value and to generate a narrow pulse signal having an ON time which is independent of the feedback signal in the low voltage state, and to.
 11. An insulated DC/DC converter comprising the primary side controller of claim
 1. 12. An isolated DC/DC converter, comprising: a transformer including a primary winding, a secondary winding, and an auxiliary winding; a switching transistor connected to the primary winding; a secondary side rectifier circuit configured to rectify a current flowing through the secondary winding to generate an output voltage; a photocoupler including a light emitting element and a light receiving element; a feedback circuit configured to drive the light emitting element of the photocoupler with a current corresponding to a difference between the output voltage and a target value; and a primary side controller configured to drive the switching transistor based on a feedback signal corresponding to a current flowing through the light receiving element of the photocoupler, wherein the primary controller is configured to drive (i) the switching transistor depending on a first pulse signal which has a first period and an ON time of the first pulse signal is adjusted depending on the feedback signal when a voltage across the auxiliary winding is greater than a predetermined value, and drive (ii) the switching transistor depending on a second pulse signal which has a second period longer than the first period and an ON time of the second pulse signal is adjusted depending on the feedback signal when the voltage across the auxiliary winding is smaller than the predetermined value.
 13. An electronic device, comprising: a load; a diode rectifier circuit configured to full-wave rectify a commercial AC voltage; a smoothing capacitor configured to smooth an output voltage of the diode rectifier circuit to generate a DC input voltage; and the DC/DC converter of claim 11, configured to step down the DC input voltage and supplies the same to the load.
 14. A power adaptor, comprising: a diode rectifier circuit configured to full-wave rectify a commercial AC voltage; a smoothing capacitor configured to smooth an output voltage of the diode rectifier circuit to generate a DC input voltage; and the DC/DC converter of claim 11, configured to step down the DC input voltage and supplies the same to a load.
 15. A method of controlling a switching transistor on a primary side of an isolated DC/DC converter, comprising: comparing an output voltage of the DC/DC converter with a predetermined value; generating a first pulse signal having an ON time which is adjusted depending on a feedback signal from a secondary side of the isolated DC/DC converter, the first pulse signal having a first period when the output voltage is higher than the predetermined value; generating a second pulse signal having an ON time which is adjusted depending on the feedback signal from the secondary side, the second pulse signal having a second period longer than the first period when the output voltage is lower than the predetermined value; and driving the switching transistor depending on the first or second pulse signal.
 16. A method of controlling a switching transistor on a primary side of an isolated DC/DC converter, comprising: comparing an output voltage of the DC/DC converter with a predetermined value; generating a first pulse signal having an ON time which is adjusted depending on a feedback signal from a secondary side of the isolated DC/DC converter, the first pulse signal having a first period when the output voltage is higher than the predetermined value; generating a second pulse signal having an ON time which is adjusted depending on the feedback signal from the secondary side, the second pulse signal having an OFF time which is longer than the first period when the output voltage is lower than the predetermined value; and driving the switching transistor depending on the first or second pulse signal.
 17. A method of controlling a switching transistor on a primary side of an isolated DC/DC converter, comprising: comparing an output voltage of the DC/DC converter with a predetermined value; generating a set signal which defines a timing at which the switching transistor is turned on; generating a reset signal which is dependent on a feedback from a secondary side and which defines a timing at which the switching transistor is turned off; generating a pulse signal depending on the set signal and the reset signal; driving the switching transistor depending on the pulse signal; and increasing an interval of the set signal when the output voltage is lower than the predetermined value. 